The invention relates to a force sensor which is integrated into an actuator for generating or transmitting a force in the force flux and has an actuator bottom that is transverse to the force flux.
In motor vehicles, the braking function is implemented nowadays by means of hydraulically activated actuators. In the method designated as xe2x80x9cbrake-by-wirexe2x80x9d, electrically operated braking devices are used. In said braking devices, actuators, (i.e. elements in which the braking force is generated and by means of which the braking force is transmitted), are activated by means of electromotive step-down drives. The resulting advantages are the individual and variable configuration of the braking process and the possibility of simultaneously performing further functions, for example the ABS function (Anti-lock Braking System). As an electromechanical braking system will generally operate with controlled braking force, the precise measurement of the braking force is essential to the performance of the overall system. High demands are made on the precision of the system due to the synchronous operation. For example, fault tolerances should be  less than 1% even if the braking force is, for example, 5 t. The measuring task is additionally made considerably more difficult as a result of limited accessibility to relevant measurement locations, the small amount of free space in the direction of force and the extremely high spatial, and chronological temperature gradients. These aspects rule out the use of known force sensors such as, for example, strain gauges.
The present invention is based on the object of enabling precise and unambiguous sensing of braking forces as close as possible to the location where the braking force acts. More specifically, the invention is based on the recognition that a force sensor can easily be integrated into an actuator. The deflection of an actuator bottom or of a braking piston bottom (a designation by analogy with hydraulic systems) can be used as a measurement variable for the braking force. The actuator bottom is appropriately configured for this purpose. The actuator is generally constructed in the form of a hollow cylinder, having an actuator bottom, and also containing a supporting ring by which it bears directly or indirectly on the brake lining of a brake. The braking force is generated centrally and applied to the actuator bottom.
The deformation of the actuator bottom is advantageously determined by means of various measuring methods. One method which is suitable for series production is the capacitive measuring method, whereby the actuator bottom constitutes an electrode of a capacitor and the capacitance which is changed with the deformation is determined. The capacitor will therefore generally be a plate capacitor. The electrode which lies opposite the actuator bottom is embodied as a plate and which is pressed onto a base with spring support so that the high temperature gradients do not cause any mechanical stresses to be transmitted to the insulator of the electrode. Thus this ensures a defined electrode spacing, as is described in the European patent EP 0 849 576 B1.
The connecting point between the actuator bottom, namely the rear part of the actuator which is generally of cylindrical construction, and the supporting ring, is embodied so as to be relatively rigid, since the braking force can cause torques to be transmitted to the supporting ring at this point and said torques cause the measurement to be subject to a hysteresis due to friction effects. For this reason, the material cross section at this connecting point is advantageously reduced by an internal peripheral groove, an external peripheral groove, or by means of a combination thereof so that only minimum torques are transmitted.
The measures which are provided for minimized hysteresis are likewise suitable for suppressing in the axial direction a temperature gradient in the actuator bottom due to largely radial introduction of heat. Axial temperature gradients can cause the actuator bottom to bulge in the direction of the force to be measured and result in an incorrect measurement.